The invention relates to a control method for jerk-limited velocity control of a moving machine element of a numerically controlled industrial processing machine such as a machine tool, a robot or the like, a trajectory of the machine element being resolved into directly sequential movement sections which can be interpolated, and to a corresponding numerical control.
Jerk limitation currently forms part of the prior art for the velocity control of a numerical control, e.g. for a machine tool or a robot. The aim thereby is to achieve a lower loading of the individual axes of a machine without encountering grave disadvantages in the program processing time.
However, the smoothing action of jerk limitation depends strongly on the required desired value characteristic. Measurements and theoretical investigations have shown that higher-frequency acceleration processes and braking processes must be carried out with a relatively low level of dynamics in order not to excite the oscillating mechanics too strongly. This functional dependence holds both in the case of short positioning operations, and in path control operation.
At present, as a remedy for this problem, the limiting value for jerking and acceleration is set so low that even high-frequency changes in the tool path feedrate do not excite the oscillations too strongly. However, these low set dynamic values often undesirably prevent a higher tool path feedrate, and thus a shorter program processing time.
A known method for obtaining a machine-protecting velocity profile is jerk limitation with the aid of a cubic polynomial, as shown in FIG. 1 and FIG. 2 of the drawings. FIG. 1 shows the tool path feedrate V of a moving machine element against time t, whilst FIG. 2 shows the associated acceleration characteristic a. Traveling employs the maximum permissible acceleration aLim in phase 1. In phase 2, traveling employs constant further acceleration at aLim, the velocity continues to increase only linearly. The acceleration a is decreased in phase 3, that is to say braking is performed. The term acceleration as used below always includes the possibility of negative acceleration for braking purposes.
Consequently, at the end of phase 3, a maximum permissible tool path feedrate vLim is reached, which is then used for traveling in phase 4. In phases 5 to 7, the velocity v is now decreased analogously such that the tool path feedrate v is zero when the desired position is reached. For this purpose, a negative acceleration is built up in phase 5 and is kept constant in phase 6, finally being reduced again to zero in phase 7. The gradient of the acceleration in phases 1, 3, 5 and 7 is critical with reference to jerk limiting of a machine. In these phases, the velocity characteristic exhibits a polynomial variation, while the velocity varies linearly in the remaining phases.
This type of jerk limitation is distinguished chiefly in that it leads to only a slight lengthening of the program processing time by comparison with acceleration-limited velocity control. Even a shorter processing time is sometimes possible if a higher acceleration can be set at such a machine because of the jerk limitation.
However, it is problematical in this case that such highly dynamic acceleration and braking processes on the path can lead in a specific frequency range to excitation of mechanical vibrations. The dynamics of these processes should therefore be adapted to the machine conditions.
It is therefore the object of the present invention to create a possibility of using jerk limitation to dampen critical resonant frequencies of a machine or a moving machine element.
In accordance with the present invention, this object is achieved by means of a control method for jerk-limited velocity control of a moving machine element of a numerically controlled industrial processing machine such as a machine tool, a robot or the like, with a trajectory of the machine element being resolved into directly sequential movement sections which can be interpolated. This is achieved by virtue of the fact that acceleration processes with associated jerk profiles which result during interpolation and are situated at or in a region near a critical natural frequency of the machine element or of the machine, are adapted such that a time which is traveled with a maximum permissible jerk corresponds substantially to the reciprocal of this critical natural frequency.
The object of the invention is further achieved by means of numerical control forjerk-limited velocity control of a moving machine element of a numerically controlled industrial processing machine such as a machine tool, a robot or the like, an interpolator being provided for interpolating a trajectory of the machine element, resolved into directly sequential movement sections that can be interpolated by virtue of the fact that by means of jerk limitation such acceleration processes can be adapted with the associated jerk profiles, which result during interpolation and are situated at or in a region near a critical natural frequency of the machine element or of the machine, such that a time which is traveled with a maximum permissible jerk corresponds substantially to the reciprocal of this critical natural frequency.
It has proved to be favorable in this case both for the control method and for the numerical control when jerk profiles resulting during interpolation and having a characteristic situated at or in a region near a critical natural frequency of the machine element or of the machine are determined with the aid of a velocity characteristic, determined modally in advance over a plurality of movement sections that can be interpolated, of the machine element on the trajectory thereof.
An adaptation of determined critical acceleration processes can be achieved in a particularly simple and therefore advantageous way by varying, in particular reducing, the maximum permissible jerk until the jerk time corresponds substantially to the reciprocal of this critical natural frequency.
Where a factor is prescribed with the aid of which the maximum permissible jerk is reduced as much as possible, it is possible to avoid an unnecessary reduction in the path dynamics.
An advantageous refinement of the device and of the control method of the present invention uses the minimum of the natural frequencies of all the axes participating in the trajectory of the machine element as critical natural frequency.
According to an alternative refinement, a mean value of the natural frequencies of all the axes participating in the trajectory of the machine element serves as critical natural frequency.
A further alternative embodiment is distinguished in that the critical natural frequency is produced by weighting the natural frequencies of all the axes participating in the trajectory of the machine element, weighting advantageously being performed with the aid of the contribution of a respective participating axis to the trajectory of the machine element.
When a separate adaptation of critical acceleration processes is performed for each participating axis, an axial jerk profile being analyzed by analyzing the axial velocity characteristic resulting for an assumed constant tool path feedrate of the machine element, it is also possible to take account of contour curvatures or transformations of a trajectory.
It has proved to be advantageous in this case when axial jerk profiles resulting during interpolation and having a characteristic situated at or in a region near a critical natural frequency on the relevant axis are determined modally in advance by section, a section being formed respectively by minima or maxima of the axial velocity characteristic.
This is performed according to a further advantageous refinement by virtue of the fact that in order to adapt determined critical acceleration processes the maximum tool path feedrate is reduced for each analyzed section of the axial velocity characteristic until the jerk time corresponds substantially to the reciprocal of the critical natural frequency of the respective axis.
A further improvement can be achieved when even the longest possible time over which it is possible to travel at maximum acceleration, and/or over which it is possible to travel at maximum velocity is adapted to the same frequency as the jerk time or a multiple thereof.
It has proved in this case to be advantageous for the numerical control according to the invention when the jerk limitation is configured as a linearized digital filter which supplies an associated desired jerk value at each operating point of the machine element.
The high-frequency changes in the tool path feedrate are carried out automatically with the aid of smaller jerk or acceleration values by using the above-described adapting of the dynamics in accordance with the present invention. This additionally permits the dynamics limiting values to be increased, thereby permitting a gain in the program processing time to be obtained.